1. Field of the Invention
The present invention relates to a hyperframe synchronization processing apparatus and a hyperframe synchronization method, and particularly relates to a hyperframe synchronization processing apparatus and a hyperframe synchronization method capable of establishing a hyperframe synchronization in a short time after establishing a frame synchronization.
2. Related Background Art
There is a communication method in which a certain fixed period of time is set as a minimum unit (a frame) of transmission of data and meaningful data are composed by a combination of a plurality of minimum units (a hyperframe). An example of such a communication method is the ADSL (Asymmetric Digital Subscriber Line) Modem Standard (G.992. 1, G.992. 2) recommended by the ITU-T.
In such a communication method, in initialization processing before starting data exchange, frame synchronization processing of establishing frame synchronization between a transmitter and a receiver is performed, and thereafter hyperframe synchronization processing of establishing hyperframe synchronization is performed. The establishment of hyperframe synchronization means that the receiver recognizes the position of a frame during reception in a hyperframe. This hyperframe synchronization is performed after the establishment of frame synchronization.
In a specification for Japan (Annex C) which is one of annexes of the aforesaid ADSL Modem Standard, a subscriber who is a receiver is obligated to perform hyperframe synchronization for a station which is a transmitter.
FIG. 27 is a diagram showing the configuration of a hyperframe of a down stream in this ADSL Modem Standard. Incidentally, the down stream means the flow of data from the station to the subscriber. The opposite direction is called an up stream. In the ITU-T Standard, hyperframe synchronization is prescribed only in the down stream.
As shown in FIG. 27. the length of one frame is 250×(68/69) μ seconds, that is, about 246μ seconds. Numbers assigned to respective frames show frame numbers in a hyperframe, and the respective frames are transmitted in order of these frame numbers. One hyperframe is composed of 345 consecutive frames, and its length is 250×(68/69)×345μ seconds, that is 85 m seconds.
In FIG. 27, a hatched frame is called an FEXT frame (far end cross talk frame), and a white frame is called a NEXT frame (near end cross talk frame). In the hyperframe, there are no other types than these. In the stage of initialization of the down stream, included in each frame is only information on distinction between an FEXT frame and a NEXT frame.
The reason why frames are divided into FEXT frames and NEXT frames as described above is that a crosstalk between this ADSL and the ISDN is taken into consideration. Namely, in view of noises in ISDN, frames can be divided into FEXT frames capable of transmitting many data and NEXT frames incapable of transmitting many data. Synchronization can be established only at every 345th frame between the ADSL and the ISDN. Hence, one hyperframe is composed of 345 frames.
Each frame of this hyperframe is composed of sin waves of 207 kHz and 276 kHz, and distinction between an FEXT frame and a NEXT frame is made by a phase of a sin wave of 207 kHz out of these sin waves. Therefore, when the receiver performs hyperframe synchronization, the receiver needs to judge where a frame during reception is in the hyperframe from the order of sequences of transmitted FEXT frames and NEXT frames.